Some bacteria contain genetic systems that may help the bacteria compete with other bacteria in the environment. One of these systems is called contact-dependent growth inhibition (CDI), in which one bacterium can inhibit growth of another bacterium after contact between the bacteria. These systems have been described in some Gram-negative genera (i.e., in some α, β and γ proteobacteria) and in a single Gram-positive species, Bacillus subtilis. 